High emissions of NO.sub.x occur in the exhaust gases in internal combustion engines which are driven with a high excess of air. This is especially true for internal combustion engines having direct injection into the combustion chamber. A reduction catalytic converter, for example, can be used for reducing these emissions. In order to reduce the NO.sub.x, fuel is metered in additionally forward of the catalytic converter. This additional fuel is converted in the catalytic converter.
Furthermore, the exhaust gases of internal combustion engines of this type are after-treated via oxidation catalytic converters. In this way, the hydrocarbon (HC) emissions are significantly reduced.
In the United States, there has for some time been the requirement for monitoring the vehicle components and to display possibly occurring faults utilizing on board diagnosis (OBD).
Catalytic converters of internal combustion engines must also be monitored in the context of this on board diagnosis.
This occurs, for example, with hydrocarbon sensors (HC sensors).
Hydrocarbon sensors of this type essentially include a heater resistor and the actual sensor element. A temperature suitable for the measuring effect is adjusted with the aid of the heater resistor. It is necessary to measure the temperature because the temperature is to be adjusted via control. This takes place, for example, via the measurement of the internal resistance of the sensor element. This measurement is carried out with alternating current so that the direct-voltage signal of the probe remains essentially unaffected.
Such a temperature measurement is disclosed, for example, in U.S. Pat. No. 4,419,190 incorporated herein by reference.
Hydrocarbon sensors of this kind are used for monitoring and controlling exhaust-gas relevant components such as catalytic converters and/or metering pumps. For this reason, their function must also be monitored. If the sensor is defective, then, for example, the additional metering of fuel, which is dependent upon hydrocarbon emissions, must be switched off for an NO.sub.x catalytic converter.